Prohibited by administrator (V 6)

I can recieve e-mails without any problem, but when I try and send them to addresses I have used previously I get a message saying "Prohibited by administrator". I know there is nothing wrong with the recipients address because I can use hotmail to the same address. I have been into the tools menu to see if there are any e-mail address selected as blocked, but there are no entries in the relevant field. Has anyone come across this before & how did they cure it. I also use incredimail and the same problem exists there.